Fuel preparation apparatus and method

ABSTRACT

An apparatus and method for fuel preparation for example by milling and drying to produce a pulverous fuel supply are described. The apparatus includes a fuel preparation unit adapted to receive a mixture of fuel and a gas and to prepare the fuel for combustion in a pulverous state; an output conduit defining an output flow path for a mixture of pulverous fuel and gas from the fuel preparation unit; a phase separator disposed to receive the mixture from the output conduit and to separate the mixture into a gas phase comprising at least a major part of the gas from the mixture and a fuel phase comprising the pulverous fuel; a gas phase conduit defining a flow path for the gas phase from the separator; a heat exchanger preferably being a process fluid heat exchanger such as a feed water heat recovery heat exchanger fluidly connected to the gas phase conduit and adapted to receive and dry the gas phase. The method applies the principles embodied in the apparatus.

The invention relates to a fuel preparation process used on power plantsfor the preparation for example by milling and drying of pulverous fueland in particular carbonaceous fuel such as pulverised coal and to anapparatus embodying the process. The invention is in particular appliedin the context of milling and drying of pulverous fuel for supply as afuel source to a thermal power plant. The invention is in particularapplied to the process for fuel milling and drying for lignite powerplants or biomass power plants. Additionally the invention is relevantto waste to energy plants.

The process aims to improve the standard/conventional process for thepreparation for example by milling and drying of coal for coal powerplants.

The process aims to improve the standard/conventional process for fuelpreparation and drying applied for biomass power plants.

The process aims to improve the standard/conventional process for fuelpreparation for waste to energy power plants.

In one conventional approach the mill dries the coal and then theproduct is directly transported to the combustion furnace. There are twosignificant disadvantages that are present, if the mills (for examplefan-beater mills) are used for processing lignite, that impactsignificantly the power plant process efficiency.

Firstly, in such a conventional process the mill dries the fuel and thenthe product is directly transported to the combustion furnace. Thisapproach is proven in the industry. However it has reduced processefficiency because of the water vapour that has been created by dryingthe fuel and passed to the combustion furnace. This vapour increases theInduced Draft (ID) fan power required to extract the flue gas from thefurnace and pass it to the chimney. Secondly, the heat consumed bymoisture evaporation is irreversibly lost.

In another conventional approach the fuel processed by the mill is heldin intermediate storage silo, from which it is transported to thefurnace. In this process arrangement the water vapour does not enter thefurnace, and therefore the plant efficiency is increased, however theheat consumed by the mill for drying the fuel is still lost.

Current efforts to improve lignite plant efficiency are focused onpre-drying the coal, and removing the moisture prior to its introductionto the milling system. Therefore the plant efficiency is increased bymaximum amount at the expense of significant investment in the dryingsystem. Such systems are present on the market and are tested on variousfull size power plants across the world (i.e. WTA or DryFining processwith fluidized bed dryers). The known processes based on fluidised beddryers require an additional milling stage to prepare the coal,otherwise the fluidised phenomenon will not be achieved. This furtherincreases the capital investment of the system.

The invention seeks to provide a process methodology that addressesinitial problems and potentially increases plant efficiency whilstlimiting and ideally avoiding the use of capital expensive pre-dryingtechnology.

In accordance with the invention in a first aspect, a method ofpreparation of a fuel to produce a pulverous fuel supply, in particularfor supply as fuel to a steam generator such as a boiler for example ofa thermal power plant comprises the steps of:

supplying a mixture of fuel and a gas to a fuel preparation unit adaptedto prepare the fuel for combustion in a pulverous sate;

outputting from the fuel preparation unit a mixture of pulverous fueland gas;

separating the mixture into a gas phase comprising at least a major partof the gas output and a fuel phase comprising the pulverous fuel;

passing the gas phase to a heat exchanger preferably being a processfluid heat exchanger such as a feed water heat recovery heat exchangerto dry the gas phase.

The fuel preparation unit is preferably a fuel pulveriser such as a milladapted to receive a mixture of fuel and a gas and to mill the fuel to apulverous state. The apparatus in such case is therefore a method ofmilling and drying of a fuel to produce a pulverous fuel supply, inparticular for supply as fuel to a steam generator such as a boiler forexample of a thermal power plant comprises the steps of:

supplying a mixture of fuel and a gas to a mill;

outputting from the mill a mixture of pulverous fuel and gas;

separating the mixture into a gas phase comprising at least a major partof the gas output from the mill and a fuel phase comprising thepulverous fuel;

passing the gas phase to a process fluid heat exchanger such as a feedwater heat recovery heat exchanger to dry the gas phase.

In accordance with the invention in a second aspect, an apparatus forthe preparation of fuel for example by milling and drying to produce apulverous fuel supply, in particular for supply as fuel to a steamgenerator such as a boiler for example of a thermal power plantcomprises:

a fuel preparation unit adapted to receive a mixture of fuel and a gasand to prepare the fuel for combustion in a pulverous state;

an output conduit defining an output flow path for a mixture ofpulverous fuel and gas from the fuel preparation unit;

a phase separator disposed to receive the mixture from the outputconduit and to separate the mixture into a gas phase comprising at leasta major part of the gas from the mixture and a fuel phase comprising thepulverous fuel;

a gas phase conduit defining a flow path for the gas phase from theseparator;

a heat exchanger preferably being a process fluid heat exchanger such asa feed water heat recovery heat exchanger fluidly connected to the gasphase conduit and adapted to receive and dry the gas phase.

The fuel preparation unit is preferably a fuel pulveriser such as a milladapted to receive a mixture of fuel and a gas and to mill the fuel to apulverous state. The apparatus in such case is therefore an apparatusfor milling and drying of a fuel to produce a pulverous fuel supply, inparticular for supply as fuel to a steam generator such as a boiler forexample of a thermal power plant comprises:

a mill adapted to receive a mixture of fuel and a gas and to mill thefuel to a pulverous state;

a mill output conduit defining an output flow path for a mixture ofpulverous fuel and gas from the mill;

a phase separator to separating the mixture into a gas phase comprisingat least a major part of the gas output from the mill and a fuel phasecomprising the pulverous fuel;

a gas phase conduit defining a flow path for the gas phase from theseparator;

a process fluid heat exchanger such as a feed water heat recovery heatexchanger fluidly connected to the gas phase conduit to receive and drythe gas phase.

The apparatus of the second aspect of the invention is an apparatus toimplement the process of the first aspect, and preferred embodiments ofeach aspect will be understood from the following discussion.

The gas phase is used for example to dry and/or transport and/orfacilitate the storage of the combined fuel and gas phase. The gas phasebecomes laden with water vapour. The invention separates at least amajor part of the water vapour laden gas phase that has been created bythe milling and drying and/or transport and/or storage of the fuel. Thewater vapour is not passed to the combustion furnace. This avoids thepenalty of increased ID fan power demand. The invention uses a heatexchanger preferably being a process fluid heat exchanger such as a feedwater heat recovery heat exchanger downstream of the phase separator toeffect condensation of vapour moisture. Latent heat consumed by thedrying process is in the preferred case recovered and transferred to aprocess fluid for recovery of sensible heat for use in an industrialprocess, and in particular is used to preheat feed water. This furtherincreases the process efficiency.

In the preferred case the invention is applied to a combined milling anddrying apparatus and method for the milling and drying of coal of highmoisture content, and examples are discussed hereinbelow in such acontext. However the skilled person will appreciate that this inventioncan be applied to both coal milling and drying systems and othermilling/fuel preparation and drying systems presenting similar problemsso that the increase of efficiency of a power cycle can be achievedwithout dedicated pre-drying facility and the examples will beinterpreted and the principles of the invention understood accordingly.

In an example combined milling and drying apparatus and method themilled fuel is dried by a gas supplied in mixture with the fuel to bemilled. Preferably the gas is at elevated temperature above ambient. Itmay be a mixture of optionally preheated air and flue gas from acombustion chamber. In such case in the method the step of supplying amixture of fuel and a gas to a mill comprises the supply of a gas atelevated temperature and optionally the step of supplying a gas atelevated temperature comprises the supply of a mixture of optionallypreheated air and flue gas from a combustion chamber, for example acombustion chamber of a steam generator.

Preferably an apparatus in accordance with the invention includes a fuelsource and a gas source together configured to supply a mixture of fueland gas to the mill. Preferably the gas source is adapted to supply agas at elevated temperature. For example the gas source comprises asource of optionally preheated air and a supply of flue gas from acombustion chamber, for example a combustion chamber of a steamgenerator.

In accordance with the invention, at least a major part of the watervapour laden gas phase and of the water vapour laden gas output from themill that has been created by the milling and drying process is divertedaway as the gas phase supplied to the heat exchanger, which effectscondensation of the water vapour and in the preferred case therebyrecovers at least some of the latent heat from the drying andevaporation process. In a preferred case substantially all of the watervapour laden gas output is diverted away as the gas phase supplied tothe heat exchanger. In a preferred case the phase separator is adaptedto separate substantially all of the gas output from the mill into thegas phase.

A particular virtue of the invention is that it is potentiallyapplicable to a range of milling technologies. The method of the firstaspect of the invention and the apparatus of the second aspect of theinvention includes a milling step or apparatus in which the fuel ismilled to a pulverous state for drying and supply for combustion orstorage. Any suitable mill may be envisaged for use in accordance withthe invention to mill the fuel to a pulverous state, for exampleincluding without limitation ball mills, horizontal and vertical rollermills, beater mills etc.

In a possible embodiment, the mill comprises a fan beater mill, forexample comprising in familiar manner a housing for receiving fuel to bepulverised, a rotary milling shaft, a beater formation carried on theshaft to effect pulverising of fuel within the housing, and an impeller.

The mill in accordance with the invention may be adapted for direct orindirect supply of pulverous fuel to a combustion chamber, for example acombustion chamber for a steam generator such as a boiler for example ofa thermal power plant.

The apparatus and method of the invention is in particular an apparatusand method for the processing of carbonaceous fuel having relativelyhigh fuel moisture content prior to milling, for example at least 25%,for example for use in a combustion apparatus. The apparatus and methodof the invention is in particular an apparatus and method for theprocessing of low-rank fuel, for example for use in a combustionapparatus. The invention in particular incorporates a carbonaceous fueland for example low-rank fuel drying process and system.

Low-rank fuels are characterised by higher fuel moisture content(typically 25-60% or more). To avoid the inefficiencies inherent incombustion of fuel with a high moisture content, the low-rank fuel ismilled to a pulverous state and dried. In a preferred case, moisturecontent may be reduced to around 10-20%.

The present invention relates to the processing by milling and drying,especially for combustion, of carbonaceous fuel having relatively highmoisture content prior to milling, for example low-rank fuels, whichterm is used herein to refer to those fuels, including fuels sometimescalled peat, lignites, brown coals or sub-bituminous coals, or biomass,which have a higher fuel moisture content (typically 25-60% or more)than bituminous coals. Thus, in the preferred case, the apparatus of theinvention comprises a supply of such fuel for milling to pulverous form,and the method of the invention comprises the milling of such asrequired by the combustion furnace.

Additionally this invention may be applied to waste combustion plants.

In accordance with the invention a heat exchanger preferably being aprocess fluid heat exchanger is fluidly connected to the gas phaseconduit downstream of the phase separator to receive and dry the gasphase. Latent heat consumed by the mill for drying or otherwise in thegas phase is recovered and transferred to heat the process fluid.

The process fluid may be a process liquid or air or other gases. Theprocess liquid may be any liquid or combination of liquids useable forheat exchange, including water, ammonia, alcohols, hydrocarbons and thelike. Preferably, the process liquid is wholly or substantially water,optionally including one or more additives or other minor componentsknown in the art.

The process fluid stream may be heated by any direct or indirect thermalexchange, method, device, unit or apparatus.

In a possible embodiment of the present invention, the process fluid iswater and/or steam and is for example water and/or steam for/from asteam generator or boiler of a steam generating process. In a possibleembodiment of the present invention, the process fluid is feedwater fora steam generator. Such a steam generator may be a boiler, optionallycomprising one or more boilers, and optionally including an integralsteam generator economiser known in the art.

Latent heat consumed by the mill for drying or otherwise in the gasphase is in this case recovered and used to preheat feed water by meansof the heat exchanger.

Such feedwater may be provided directly or indirectly from a feedwaterstream. Preferably, a portion of such a feedwater stream is provided asthe process liquid for the system of the present invention. Such aportion may be provided as a slip stream of such a feedwater stream,such a slip stream generally being a minor portion of the full feedwaterstream.

Thus in such a case the heat exchanger of the apparatus of the inventionis preferably a process fluid heat exchanger and is for example a heatexchanger disposed to exchange heat with a process fluid and supply thesame for use in an industrial process. In particular the heat exchangeris adapted to supply the process fluid to an industrial process in suchmanner as to make use of the sensible heat therein. For example theprocess fluid is feedwater for a steam generator and the the processfluid is supplied in such manner as to make use of the sensible heatrecovered in the heat exchanger is effected in that the sensible heatrecovered in the heat exchanger effects preheating of the feedwater.

By analogy in the method of the invention the heat exchanger is aprocess fluid heat exchanger and the method comprises supply of processfluid to the heat exchanger. In particular the heat exchanger is a heatexchanger disposed to exchange heat with a process fluid for use in anindustrial process and the method comprises using the heat exchanger toeffect condensation of the water vapour and thereby recover at leastsome of the latent heat from the drying process. Preferably the methodcomprises the further step of using the process fluid in an industrialprocess in such manner as to make use of the sensible heat recovered inthe heat exchanger. In a particularly preferred case the process fluidis feedwater for a steam generator and the step of using the processfluid in such manner as to make use of the sensible heat recovered inthe heat exchanger, effected in that the sensible heat recovered in theheat exchanger effects preheating of the feedwater.

The heat exchanger may be any suitable form of direct or indirect heatexchanger of familiar or bespoke design. The heat exchange is in thepreferred case a process fluid heat exchanger from which sensible heatcan be recovered for utilisation within an industrial process, and forexample within a steam generation process. In a possible more completeaspect of the invention, the method comprises a method of milling anddrying of a fuel to produce a pulverous fuel supply in accordance withthe first aspect of the invention, and further comprises the additionalstep of supplying the fuel to a combustion apparatus. A combustionapparatus is for example a combustion chamber of a steam generator suchas a boiler, for example comprising the boiler of a thermal power plant.

In a possible embodiment, the fuel phase is supplied directly to theburners. In another possible embodiment, the fuel phase is suppliedindirectly for example via a storage silo.

In an alternative more complete aspect of the invention, the methodcomprises a method of milling and drying of a fuel to produce apulverous fuel supply in accordance with the first aspect of theinvention, and the further step of passing the dried fuel for storage.

The method comprises passing the gas phase to a heat exchangerpreferably being a process fluid heat exchanger to dry the gas phase,and preferably further comprises using recovered sensible heat from theheat exchanger, for example as a means of heating a process fluid whichmay be a liquid, gas or mixture, in an additional industrial process.The additional industrial process is for example a process of operationof a thermal power plant. The process fluid in such a case is forexample a process fluid which may be a liquid, gas or mixture used inthe operation of a thermal power plant, and is for example feedwater orair.

By analogy, in accordance with a further more complete aspect of theinvention, an apparatus is provided for milling and drying of a fuel toproduce a pulverous fuel supply, which apparatus further comprises acombustion apparatus and a fuel supply conduit to supply the fuel phasecomprising the pulverous fuel to the combustion apparatus. Thecombustion apparatus is for example the combustion chamber of a steamgenerator such as a boiler for example of a thermal power plant and thesupply conduit is adapted to supply the fuel phase comprising thepulverous fuel to a combustion chamber within the steam generator.Optionally the supply conduit is configured to supply the fuel phasedirectly to one or more burners of the steam generator. Alternativelythe supply conduit is configured to supply the fuel phase to a storagesilo for example being a storage silo of an indirectly fired steamgenerator.

In accordance with an alternative further more complete aspect of theinvention, an apparatus in accordance with any preceding claim furthercomprises a storage volume and a supply conduit to receive the fuelphase and supply the fuel phase to the storage volume.

The invention will now be described by way of example only withreference to FIGS. 1 and 2 of the accompanying drawings in which:

FIG. 1 is an example system for the supply of pulverous fuel directly tothe burners for combustion in combustion furnace embodying theprinciples of the invention;

FIG. 2 is an example system for the indirect supply of pulverous fuelembodying the principles of the invention.

The following discussion considers processes applied in particular to astandard state of the art milling system such as a fan-beater mill usedfor processing fuel for combustion into pulverous form. It will beappreciated that this is an example only. FIG. 1 shows the supply ofpulverous fuel directly to the burners of a combustion furnace and FIG.2 shows the indirect supply of pulverous fuel to a combustion furnace.To the extent that features of the embodiment of the invention and ofthe milling system and combustion furnace are common, like referencenumerals are used.

The proposed system is used in conjunction with the state of the artmilling system such as a fan-beater mill. The process incorporates theuse of a feed water heat recovery heat exchanger to dry the gas phase offan-beater mill product stream and thereby also to recover at least someof the latent heat input from the drying process.

Fuel for combustion, for example comprising a carbonaceous fuel ofrelatively high moisture content such as a lignite or other low-rankfuel, is delivered from a fuel supply 2 and combined with a hot gasstream comprising a mixture of cold or preheated air 4 and flue gas 6.The mixture is supplied to a beater mill 8 to be milled into pulverousform suitable for combustion. The hot gases within the mill dry the fuelto a more suitable state for combustion. For example a moisture contentof 25 to 60% is reduced to 5 to 10%. Water vapour from the fuel passesinto the gas phase. However if the product of the mill is directlytransported to the combustion furnace including the water vapour thathas been created by drying the fuel this may reduce the processefficiency because the water vapour that has been created by drying thefuel is passed to the combustion furnace.

Instead, the stream that leaves the beater mill is first separated atthe phase separator 12 into a pulverised and dried fuel phase, and a gasphase. Secondly, the gas phase is passed through a feed water heatexchanger 10 where the condensation of vapour moisture is carried out.This enables the plant to reduce the ID fan power demand, hence increasepower plant energy sales. Additionally the process efficiency increasesas the gas leaving the power plant will have less moisture and thereforea smaller heat capacity, what will result in less heat being lostthrough the chimney. Additionally the process efficiency increases asthe latent heat consumed by the mill for drying is recovered andtransferred to pre-heat the feed water. Importantly, this will reducethe fuel consumption while the power plant output will remain unchanged.

The pulverised fuel phase, substantially denuded of the gas and watervapour mixture, is passed to the burners 13 of a combustion furnace 14of a steam generator. In FIG. 1 the pulverised fuel phase is supplieddirectly. FIG. 2 is an example system for the indirect supply ofpulverous fuel to a silo 11 for onward supply to the burners 13 of acombustion chamber 14 of a steam generator. The steam generator may beon any suitable conventional or bespoke design.

The process calculation of the described solution, shows that assumed50% latent heat recovery in the system the power plant net efficiencywill increase by 2.65% p, and if 90% of heat recovery is achieved, theapplication of the invention will improve the net efficiency by 3.84% p.The amount of heat recovery is driven by the economic factors thatinfluence the design to increase cost effectiveness of the system.

The dried fuel product separated from the gas phase in separator ispassed directly to the burners for combustion in combustion furnace aspresented on FIG. 1, or is safely stored in intermediate storage device(i.e. silo) and then transferred to the burners for combustion aspresented on FIG. 2.

In another application current invention could be used to dry the rawfuel and store the product for transportation (i.e. shipping) andcombustion on remote location.

1. An apparatus for preparation and drying of a fuel to produce a pulverous fuel supply comprising: a fuel preparation unit adapted to receive a mixture of fuel and a gas and to prepare the fuel for combustion in a pulverous state; an output conduit defining an output flow path for a mixture of pulverous fuel and gas from the fuel preparation unit; a phase separator disposed to receive the mixture from the output conduit and to separate the mixture into a gas phase comprising at least a major part of the gas from the mixture and a fuel phase comprising the pulverous fuel; a gas phase conduit defining a flow path for the gas phase from the separator; a heat exchanger fluidly connected to the gas phase conduit to receive and dry the gas phase.
 2. An apparatus in accordance with claim 1 wherein the fuel preparation unit is a mill adapted to receive a mixture of fuel and a gas and to mill the fuel to a pulverous state.
 3. An apparatus in accordance with claim 1 wherein the heat exchanger is a process fluid heat exchanger.
 4. An apparatus in accordance with claim 1 wherein the heat exchanger is a heat exchanger disposed to exchange heat with a process fluid and supply the same for use in an industrial process.
 5. An apparatus in accordance with claim 4 wherein the heat exchanger is adapted to supply the process fluid to an industrial process in such manner as to make use of the sensible heat therein.
 6. An apparatus in accordance with claim 4 wherein the process fluid is feedwater for a steam generator.
 7. An apparatus in accordance with claim 1 wherein the phase separator is adapted to separate substantially all of the gas output from the mill is separated into the gas phase,
 8. An apparatus in accordance with claim 1 further comprising a fuel source and a gas source together configured to supply a mixture of fuel and gas to the mill.
 9. An apparatus in accordance with claim 8 wherein the gas source supplies a gas at elevated temperature.
 10. An apparatus in accordance with claim 9 wherein the gas source comprises a source of optionally preheated air and a supply of flue gas from a combustion chamber.
 11. An apparatus in accordance with claim 8 wherein fuel source comprises a supply of a low-rank fuel with a moisture content of at least 25% on average.
 12. An apparatus in accordance with claim 1 further comprising a storage volume and a supply conduit to receive the fuel phase and supply the fuel phase to the storage volume.
 13. An apparatus in accordance with claim 1 further comprising a steam generator and a supply conduit to receive the fuel phase and supply the fuel phase to the steam generator.
 14. An apparatus in accordance with claim 13 wherein the supply conduit is configured to supply the fuel phase directly to one or more burners of the steam generator.
 15. An apparatus in accordance with claim 14 wherein the supply conduit is configured to supply the fuel phase to a storage silo of a steam generator.
 16. A method of preparation of a fuel to produce a pulverous fuel supply comprising the steps of: supplying a mixture of fuel and a gas to a fuel preparation unit adapted to prepare the fuel for combustion in a pulverous sate; outputting from the fuel preparation unit a mixture of pulverous fuel and gas; separating the mixture into a gas phase comprising at least a major part of the gas output and a fuel phase comprising the pulverous fuel; passing the gas phase to a heat exchanger to dry the gas phase.
 17. A method in accordance with claim 16 applied as a method of milling and drying of a fuel to produce a pulverous fuel supply, in that the fuel preparation unit is a mill applied to receive a mixture of fuel and a gas and to mill the fuel to a pulverous state.
 18. A method in accordance with claim 16 wherein the heat exchanger is a process fluid heat exchanger.
 19. A method in accordance with claim 16 wherein the heat exchanger is a heat exchanger disposed to exchange heat with a process fluid for use in an industrial process and the method comprises using the heat exchanger to effect condensation of the water vapour and thereby recover at least some of the latent heat from the drying process.
 20. A method in accordance with claim 19 comprising the further step of using the process fluid in an industrial process in such manner as to make use of the sensible heat recovered in the heat exchanger.
 21. A method in accordance with claim 20 wherein the process fluid is feedwater for a steam generator and the step of using the process fluid in such manner as to make use of the sensible heat recovered in the heat exchanger in that the sensible heat recovered in the heat exchanger effects preheating of the feedwater.
 22. A method in accordance with claim 16 wherein substantially all of the water vapour laden gas output from the mill is separated into the gas phase and diverted away to the heat exchanger.
 23. A method in accordance with claim 16 wherein the step of supplying a mixture of fuel and a gas to a mill comprises the supply of a gas at elevated temperature.
 24. A method in accordance with claim 23 wherein the step of supplying a gas at elevated temperature comprises the supply of a mixture of optionally preheated air and flue gas from a combustion chamber.
 25. A method in accordance with claim 16 wherein the step of supplying a mixture of fuel and a gas to a mill comprises the supply of a low-rank fuel with a moisture content of at least 25%.
 26. A method in accordance with claim 16 comprising a method of producing a pulverous fuel supply for a steam generator such as a boiler for example of a thermal power plant.
 27. A method in accordance with claim 26 further comprising the additional step of supplying the fuel phase to the steam generator.
 28. A method in accordance with claim 27 further comprising the additional step of supplying the fuel phase directly to one or more burners of the steam generator.
 29. A method in accordance with claim 28 further comprising the additional step of supplying the fuel phase to a storage silo of a steam generator.
 30. A method in accordance with claim 16 comprising the further step of passing the fuel phase for storage. 